This research aims to investigate the thoracic chemoreceptors (aortic bodies) of adult dogs. We are particularly interested in those group 4 bodies which lie between aorta and pulmonary artery and are supplied with blood by a small artery or arteries which arise from the left coronary artery or its major branches. We will perfuse the left coronary artery through a silicone rubber tipped cannula which does not require dissection. We will study the reflex changes in arterial pressure and heart rate which follow alterations in perfusion pressure, PO2, PCO2 and pH. We will compare the changes in dogs in which the pericardium is intact with those in which we have isolated the chemoreceptor blood supply by dissection at the point where injections of 5 micron g doses of serotonin or phenylbiguanide through an exploring intracoronary catheter result in maximal reflex increases in heart rate and arterial pressure. We will also study the effects of occlusion of the chemoreceptor blood supply on the reflex induced by alterations in perfusion pressure and levels of blood gases. We will investigate the role of the small vessels supplying these chemoreceptors in a reflex depressor response which follows elevation of pressure in the left coronary artery to determine whether these vessels are flow and/or pressure sensitive. We also plan to study the nerve supply of these receptors, their possible effect on coronary resistance and the effect of the permanent occlusion of their blood supply.